The present invention relates to a DC motor drive system and method for controlling motor output torque. More particularly, the invention relates to a DC motor drive system which is mechanically connected to drive a linearly-reciprocable liquid pump.
In the prior art, linearly-reciprocable liquid pumps have most often been mechanically coupled to air motor drive systems; an obvious advantage in utilizing an air motor drive system lies in the fact that an air motor can be constructed to operate in a linearly-reciprocable fashion, with movement and position matched to the characteristics of the pump. In typical systems, the linear pump is driven over a predetermined stroke distance, whereupon its direction of travel is changed and it is driven in the reverse direction over the same predetermined stroke distance. The pump typically has two internal chambers so that it may discharge liquid from one of these chambers during the length of its stroke, while at the same time it is receiving liquid into the other of its chambers; therefore the pump both discharges and receives liquid during both the forward and reverse travel of a stroke. An air motor driver may be constructed to impart driving force in the forward and reverse directions, over a stroke which is matched to the pump delivery stroke. The interval of time and distance wherein the pump changes its stroke direction is called the "changeover," and with an air motor driving system there is typically a small pressure change evident in the liquid delivered by the pump during the changeover. However, this pressure change is usually quite small, because an air motor has low inertia and is able to move through the changeover cycle fairly quickly; i.e., an air motor has very little inertia or stored energy to dissipate during the changeover. At or near the time of changeover the liquid check valves in a reciprocable pump also contribute to the pressure changes which are evident during the changeover interval. The overall changeover effect is a sharp pressure drop followed immediately by a pressure surge, but the low inertia of an air motor driver causes this pressure perturbation to be quickly corrected. However, when a reciprocable pump is driven by a torque-controlled electric motor system, the inertia of the motor system magnifies these pressure perturbations, creating a longer pressure drop and a higher pressure surge during the changeover interval. This has long been considered a disadvantage in using electric torque-controlled motor systems for this purpose.
Another advantageous feature of utilizing a linear air motor to drive a linearly-reciprocable pump is the ability of the air motor to "stall" whenever the liquid output pressure becomes sufficiently high to counterbalance the air pressure applied to the air motor. This feature is particularly important in systems wherein liquid flow may be intermittently turned on and off, such as in liquid delivery or paint delivery systems. When the liquid flow valve is suddenly shut off, the air motor will stop any further drive motion as soon as the blocked liquid pressure becomes equalized to the air motor driving pressure, and the air motor will remain in the stalled condition until such time as the liquid flow valve is again opened.
Among the disadvantages of utilizing an air motor driving system is in the fact that an external source of air pressure is required, and such systems tend to be relatively energy-inefficient. As a result of these and other disadvantages, other forms of motor drive systems have been designed for mechanically connecting to linearly-reciprocable pumping systems. Electric motor drive systems have been tried in this application, with electrical controls to regulate the motor driving speed as a function of either liquid pressure or volume flow. Electrical cut-off switches have been devised to disconnect the electrical power to such a motor when conditions of blocked pressure are encountered, and DC motor drive systems have been devised wherein the motor is permitted to go into a stall condition with a reduced level, constant current being applied to the motor to produce a motor torque which counterbalances the block pressure in the pump delivery system.
A disadvantage in the use of a DC motor to drive a linearly-reciprocable pump lies in the fact that the DC motor typically has a very high inertia or stored energy which becomes a problem during changeover of the pump from one stroke direction to the other. During pump changeover, the liquid pressure typically momentarily drops, resulting in a sudden increase in motor speed; thereafter, the increased motor speed develops an increased pressure which is felt as a pressure surge in the pump delivery system at the completion of the changeover cycle. Therefore, pressure spikes occur during each changeover cycle, which can degrade the liquid delivery characteristics which are desired in any given system. When liquid flow valves in the system are turned on and off, pressure variations inherently occur as a result of the motor inertia, resulting in pressure and flow surges which disturb the relatively smooth liquid flow characteristics desired from the system.
The present invention provides an energy management technique for controlling a DC motor drive system of the type described, to prevent over pressurization of the liquid delivery system. The invention provides an apparatus and method for driving the apparatus, which absorbs the kinetic energy of the motor rotor electrically whenever the liquid flow stops, and limits the speed increase of the motor during liquid pump changeovers. The invention also enables a reduction in power consumption of the system under blocked pressure conditions, by selectively reducing the electrical motor drive power during such conditions.